As laptop personal computers, mobile phones and electric automobiles have rapidly expanded their markets, there have been needed a charge storage device with a higher energy density and a large current. Among others, a lithium ion secondary battery wherein a positive electrode uses a lithium-containing transition metal oxide while a negative electrode uses a carbon material has been used as a high-energy secondary battery in a variety of electronic devices.
However, since the lithium-ion secondary battery has a small reaction rate in an electrode reaction, a large current leads to significant deterioration in battery performance. Therefore, when used in a small electronic device, the battery requires a longer charging time. Also, the battery has not been used in a device such as an electric automobile requiring a large current.
On the other hand, an electric double layer capacitor using charcoal in an electrode can give a large current and exhibit good cycle properties. It has been, therefore, developed as a backup power source and a power source for an electric automobile. However, since it has a small energy density and thus size reduction has been difficult, it has not been utilized in mobile electronic devices. Furthermore, it has not been extensively mounted in an electric automobile due to its poor capacity.
As with an electric double layer capacitor, a lead storage battery can give a large current and exhibit good cycle properties. It has been, therefore, widely used as an auxiliary power source for an automobile. However, since its voltage and energy density are low, it has not been applied to an electric automobile in which a motor is driven by electric power.
As described above, a lithium-ion secondary battery cannot be used for a large current and an electric double layer capacitor or lead storage battery cannot give an adequate energy density required for a small electronic device. In other words, although there have been a variety of proposals for a charge storage device which can be applied to a mobile electronic device or electric automobile, no charge storage devices with a higher energy density through which a large current can be drawn have been obtained.